Heavy metal and main group element pollution is an existing and growing worldwide problem. During the past few decades, federal and state governments have instituted environmental regulations to protect the quality of surface and ground water from contaminants. In response to these regulatory requirements, numerous products have been developed to precipitate contaminants from surface water, ground water and soil. Examples of compositions and methods utilized in precipitating metals from water and soil are detailed in U.S. Pat. No. 6,586,600, the entire disclosure of which is hereby incorporated by reference.
There are numerous industrial and environmental situations where ligands capable of binding metals and main group elements can be utilized for remediation purposes. For example, waste water issuing from waste treatment facilities, chlor-alkali industries, metal finishing industries and certain municipal landfills often present contamination problems. Similarly, the metal content of water exiting both functional and abandoned mines is a significant environmental issue in geographical areas with a heavy mining industry. Soil and surface waters located in areas near natural gas pump houses suffer a similar metal contamination problem. Gasses emitted from coal-fired power plants and the incineration of municipal and medical waste contain mercury. Thus, there is a need for ligands capable of binding and removing metals and main group elements from gasses, aqueous and non-aqueous solutions and solid substrates.
It is known in the art to use sulfur-containing compounds to bind heavy metals. For example, Thio-Red® is a chemical reagent used for precipitating divalent heavy metals from water. This product is a complex aqueous solution of sodium (with or without potassium) thiocarbonate, sulfides, and other sulfur species. Thio-Red® ultimately removes Cu, Hg, Pb, and Zn from aqueous solutions through the formation of metal sulfides (i.e. CuS, HgS, PbS, and ZnS), rather than metal thiocarbonates. Sodium and potassium dialkyldithiocarbamates such as HMP-2000®, are also widely used as metal precipitants. However, the limited ability of most reagents presently used on a commercial basis to form stable, covalent bonds with heavy metals is a major concern for remediation applications. Reagents that lack sufficient or metal-specific binding sites may produce metal precipitates that are unstable over time and under certain pH conditions. Such unstable precipitates may release bound metal back into the environment, thereby proving unsatisfactory as treatment or remediation agents. Further, these reagents may form simple metal sulfides which bacteria are capable of methylating (in the case of Hg, forming the water-soluble cation, MeHg+). Accordingly, there is a need for ligands which not only bind metals and main group elements, but also hind these elements in such a manner as to form stable, insoluble precipitates which retain the contaminant element(s) over a wide range of environmental conditions and over extended periods of time.
Likewise, it is known to use a variety of chelators for chelation therapy of metals. Many studies today reflect the increasing exposure of the population to mercury and other toxic heavy metals. Examples of currently approved binders for treating heavy metal toxicity such as mercury toxicity are dimercaptopropanesulfonate (DMPS) and dimercaptosuccinic acid (DMSA), which were introduced during World War II to combat industrial exposure to heavy metals. Conventional compounds such as DMPS and DMSA, while often referred to as “chelators,” are not truly chelators in the chemical sense of the word. This is because there is insufficient space between the sulfurs on adjacent carbon atoms to allow a large metal atom to bind to both sulfurs at the same time, which is a requirement for forming a true “chelate.” Rather, DMPS and DMSA form bound sandwich complexes with metal, where for example two binder molecules bind to a single mercury atom. This provides a weaker attachment than would be the case with a true chelator, which would form two bonds between the thiol (—SH) groups and the HG2+. Also, based on their negatively charged properties, binders like DMSA, DMPS and EDTA have a non-specific attraction for all metal ions, including the essential metals Ca2+, Mg2+, Mn2+, etc. The rapid excretion of these binders from the body through the urine can have the negative effect of depleting the body of these essential metals. Deaths have occurred by essential metal depletion by charged binding compounds during a process called chelation therapy, and this medical treatment must therefore be done by an experienced physician.
Heavy metals such as mercury are typically lipid-soluble or can pass through the cell membrane via native divalent metal ion carriers (e.g. for Ca2+, Mg2+) as the M2+ form, and may therefore concentrate intracellularly and more so in the adipose, or fatty, tissue or in other tissues high in lipid content, including without limitation the central nervous system. Indeed, mercury and other heavy metals preferentially partition to and concentrate in the hydrophobic aspects of mammals, fish, and the like, such as fatty tissues, cell membranes, lipid-containing areas of the interior of a cell, and the like.
Thus, the currently available, approved heavy metal binders have several disadvantages with regard to their overall chemical nature that could be improved on by the synthesis of better-designed, true chelators that have safer excretory properties such as higher affinity for the metals and/or main group elements and excretion through the feces instead of the urine. Such better-designed, true chelators would desirably be uncharged, lipid-soluble or hydrophobic compounds, or alternatively convertible from water soluble (for suitability for delivery via the bloodstream) to lipid-soluble compounds in the body, to allow them to partition into the fatty (hydrophobic) tissues where the mercury or other heavy metal burden is primarily located. Further, such chelators would possess low or, better yet, no observable toxicity to mammals alone in the absence of heavy metal exposures. They would be true chelators that would bind heavy metals and main group elements exceptionally tightly, preventing toxic effects and also preventing release or concentration in toxic form in any organ of the body. Still further, desirably the chelators would be excreted through the biliary transport system of the liver into the feces instead of through the kidneys (a very sensitive organ to heavy metal exposure) and into the urine. Still yet further, it would be desirable to provide improved chelators which readily convert between water-soluble and lipid-soluble forms, allowing excretion by the desired route, i.e., via the kidney for the water-soluble form and via the biliary transport system of the liver into the feces for the lipid-soluble form.